1. Field of the Invention
The present invention relates to a system for detecting the ignition range of the internal combustion engines.
A detector for detecting the ignition range of the internal combustion engine is advantageous for control of the air-fuel ratio, the EGR (exhaust gas recirculation) and ignition timing.
2. Description of the Prior Art
The importance of an ignition range detector will be described with reference to a conventional method of air-fuel ratio control.
FIG. 1 is a graph showing the relation between air-fuel ratio of the mixture supplied to a combustion chamber of the internal combustion engine, the composition of the exhaust gas and the fuel consumption rate with respect to NOx, HC and CO. If the mixture is capable of being controlled to the neighborhood of the optimum lean air-fuel ratio (the air-fuel ratio where the fuel may be completely combusted before the increase of HC by misfire) in the area (x) of FIG. 1, the harmful components CO and HC of the exhaust gas are minimized. Under this condition, the component NOx is reduced as compared with in the area at or near the generally used stoichiometric air-fuel ratio, thereby contributing to an improved purification of the exhaust gas. Further, the fuel consumption rate is lowest at or near the optimum lean air-fuel ratio, resulting in high economy.
It is therefore desirable to control the mixture gas to or near the optimum lean air-fuel ratio to attain an economically advantageous purification of the exhaust gas. In reality, however, it is difficult to maintain the air-fuel mixture at the air-fuel ratio immediately before misfire, so that the mixture gas is ignited in an area considerably richer than the optimum lean air-fuel ratio for the sake of security.
In order to solve this problem, it is necessary to develop means for detecting the air-fuel ratio immediately before misfire. Conventional means for directly detecting the air-fuel ratio of the mixture includes an air-fuel ratio detector using a solid electrolyte such as zirconium oxide or zirconia. The zirconia air-fuel ratio detector, however, has the disadvantage that it is incapable of detecting the stoichiometric air-fuel ratio but only the neighbourhood thereof (the air-fuel ratio 14.5 to 15.0 in FIG. 1).
The air-fuel ratio immediately before misfire is thus required to be detected indirectly. The optimum lean air-fuel ratio, which is one where the complete combustion is effected immediately before misfire as mentioned above, may be detected at a point slightly richer than the air-fuel ratio associated with the condition (partial combustion) immediately before misfire.
It is therefore possible to detect the air-fuel ratio by an ignition range detector for detecting the condition immediately before misfire (partial combustion).
An ignition range detector is useful as will be seen from the foregoing description about the air-fuel ratio.
The diagram of FIG. 2 shows the difference in the waveform of pressure in the cylinder of the internal combustion engine as a general according to the combustion conditions.
In FIG. 2, the in-cylinder pressures P.sub.1 (+.THETA.A) and P.sub.1 (+.THETA.B) for complete combustion at predetermined angles +.THETA.A and +.THETA.B respectively during the explosion-combustion process (expansion stroke) are larger than the in-cylinder pressures P.sub.1 '(-.THETA.A) and P.sub.1 '(+.THETA.B) respectively associated with partial combustion immediately before misfire. It is therefore possible to decide whether completion combustion or partial combustion immediately before misfire is involved by measuring the pressure in the cylinder at a predetermined angle of the internal combustion engine during the expansion process for combustion and comparing the result thereof with a predetermined value. An ignition range is accordingly capable of being detected by utilizing the result of such comparison.
The diagram of FIG. 3 shows the difference in the waveform of pressure in the cylinder of the internal combustion engine in general according to the load conditions.
Depending on the operating conditions of the internal combustion engine, the pressure Pi(+.THETA.B) is considerably different at a predetermined angle during the expansion process for completely combusted conditions. If a predetermined value for comparison (k.sub.1 =a fixed value) is determined for combustion under high load, the pressure value P.sub.2 (+.THETA.B) is smaller than the predetermined value and it is wrongly decided that partial combustion immediately before misfire is involved in spite of complete combustion under a small load. In order to solve this problem, the in-cylinder pressure P(-.THETA.A) for the fixed angle -.THETA.A before the maximum ignition advance angle (involving the compression process alone) is determined as an initial condition each time of combustion, and the value Pi(-.THETA.A) determined from this initial condition is divided by the predetermined angle Pi(+.THETA.B) during the expansion process. The result of division is compared with a predetermined value .beta., thus making it possible to decide at a predetermined level whether partial combustion immediately preceding to misfire or complete combustion is involved regardless of the load as shown in FIG. 3. In other words, the ignition range can be determined without regard to the running conditions including the load.